This post has been updated with information about TriMet MAX delays and air pollution notices.

The National Weather Service in Portland predicts temperatures in the low 90s for much of the weekend with a slight chance of a thunderstorm on Sunday. The service has also issued a heat advisory for the Portland metro area that began Saturday at 5 a.m. andclears at 2 p.m. on Tuesday, July 15.

Saturday will see a high of 91 with a low of 66. Sunday's predicted high is 88 with a low of 66 — there's a 20 percent chance of a thunderstorm during the day and a 30 percent chance in the evening.

Monday's high is predicted at 94 with another chance of thunderstorms, 20 percent.

Washington and Multnomah counties advise senior citizens to visit their local senior centers, which are equipped with air conditioning.

The county's libraries also serve this function. Here's a list of other Multnomah County cooling centers:

TriMet announced that riders should anticipate minor delays since trains are running at lower speeds in order to avoid damaging wires and rails.

The Department of Environmental Quality has issued an alert that smog levels may increase throughout the weekend and into next week due to the severity of the heat and low winds. Pollution may rise to the moderate range by Sunday and into the "unhealthy for sensitive groups" range by Monday and into the week.